First, classification of NAT (network address translation) is explained. The NAT includes the transmission port assigning rule and reception filter rule, and the NAT is classified according to their combination. The transmission port assigning rule is divided into a cone type in which, not depending on the packet destination (IP address, port), when the port of NAT local side (for example, LAN side) information processor and the IP address are the same, the port of the global side (for example, the Internet or WAN side) assigned to the NAT is the same, an address sensitive type in which a new port is assigned for each packet destination address, and a port sensitive in which a new port is assigned for each destination port of packet. The reception filter rule for judging possibility of reception of packet from the local side to the port transmitting the packet from the NAT local side is divided into an address sensitive filter for receiving the packet only from the address transmitting the packet from the port, a port sensitive filter for receiving the packet only from the port transmitting the packet from the port, and no filter having no filter. By combining the transmission port assigning rule and reception filter rule, the NAT can be classified into the following type types.
Full cone NAT: Transmission port assigning rule of cone type, and reception port filter rule of no filter.
Restricted cone NAT: Transmission port assigning rule of cone type, and reception port filter rule of address sensitive filter.
Port restricted cone NAT: Transmission port assigning rule of cone type, and reception port filter rule of port sensitive filter.
Address sensitive symmetric NAT: Transmission port assigning rule of address sensitive type, and reception port filter rule of address sensitive filter.
Port sensitive symmetric NAT: Transmission port assigning rule of port sensitive type, and reception port filter rule of port sensitive filter.
In the NAT communication, it has been proposed to establish communications between PC1 and PC2 without using server as shown in FIG. 40.
In this case, probably, there is a combination of NATs unable to establish communications. In FIG. 40, when communicating from PC1 of information processor connected to NAT1 local side to PC2, let us call NAT1 as sending side NAT, and NAT2 as receiving side NAT. Hence, combination of NATs capable of establishing communications between PC1 and PC2 is as shown in FIG. 41. Abbreviations of NATs in FIG. 41 are specifically described in the following preferred embodiments. In FIG. 41, connection of *1 has been known hitherto. Even in the combination of NATs capable of establishing communications, connection of *3 involves an uncertainty because communications cannot be established unless the port number differential of NAT is known and the position of the latest port of receiving side NAT is known.
As prior conditions for establishing communications between information processors, it is desired to detect the range of ports of NAT passing through the bubble packet (the packet sent in order to leave communication record in the NAT) transmitted from one information processor.